1. Field of the Invention
The present invention relates to crystal-oscillator circuits for phase-locked loop (PLL) circuits for use in image display devices having television tuners and radio frequency (RF) modulators.
2. Description of the Related Art
In image display devices having television tuners and RF modulators, the television tuners and the RF modulators are provided with PLL circuits. Each PLL circuit is provided with a crystal-oscillator circuit employed as a reference oscillation circuit. FIG. 3 shows the structure of a conventional crystal-oscillator circuit. The crystal-oscillator circuit includes a first crystal-oscillator circuit 20 employed for a PLL circuit of a television tuner and a second crystal-oscillator circuit 30 employed for a PLL circuit of an RF modulator. The first crystal-oscillator circuit 20 and the second crystal-oscillator circuit 30 share a single quartz crystal resonator 40.
The first crystal-oscillator circuit 20 is formed by a first oscillation transistor 21, the quartz crystal resonator 40, and the like. The second crystal-oscillator circuit 30 is formed by a second oscillation transistor 31, the quartz crystal resonator 40, and the like. In FIG. 3, DC bias circuits for the oscillation transistors 21 and 31 are omitted.
In the first crystal-oscillator circuit 20, the collector of the first oscillation transistor 21 is grounded at a high frequency by a ground capacitor 22. A feedback capacitor 23 extends between the emitter and the ground, and a feedback capacitor 24 extends between the base and the emitter. A Clapp capacitor 41 is provided in series with the quartz crystal resonator 40 between the base and the ground. The Clapp capacitor 41 is connected to the base, whereas the quartz crystal resonator 40 is connected to the ground. An oscillation signal is output from the emitter of the first oscillation transistor 21 and is input to the PLL circuit of the television tuner (not shown).
In contrast, in the second crystal-oscillator circuit 30, the collector of the second oscillation transistor 31 is grounded at a high frequency by a ground capacitor 32. A feedback capacitor 33 extends between the emitter and the ground, and a feedback capacitor 34 extends between the base and the emitter. A Clapp capacitor 42 is provided in series with the quartz crystal resonator 40 between the base and the ground. The Clapp capacitor 42 is connected to the base, whereas the quartz crystal resonator 40 is connected to the ground. An oscillation signal is output from the emitter of the second oscillation transistor 31 and is input to the PLL circuit of the RF modulator (not shown).
In the above arrangement, the single quartz crystal resonator 40 is shared by the two crystal-oscillator circuits 20 and 30. With regard to one crystal-oscillator circuit, it seems that the oscillation transistor of the other crystal-oscillator circuit is connected in parallel with the quartz crystal resonator 40. Hence, the former crystal-oscillator circuit has a significantly small apparent Q-factor of the quartz crystal resonator 40, failing to ensure a sufficient load resistance required for oscillation. This may cause oscillation to stop.
Accordingly, it is an object of the present invention to provide a crystal-oscillator circuit that prevents oscillation stoppage due to a reduction in the apparent Q-factor of a quartz crystal resonator when an oscillation signal is extracted.
To this end, a crystal-oscillator circuit according to the present invention includes an oscillation transistor having the collector or the base thereof at a ground potential. A first capacitor is connected between the base and the emitter of the oscillation transistor. A second capacitor is connected between the emitter and the collector. A quartz crystal resonator and a capacitor are connected in series between the base and the collector. An oscillation signal is extracted from a node between the quartz crystal resonator and the capacitor.
The collector of the oscillation transistor may be grounded. The quartz crystal resonator may be disposed at the base side. The capacitor may be disposed at the ground potential side.
According to a crystal-oscillator of the present invention, no other circuit is connected in parallel with the crystal-oscillator circuit. Hence, the apparent Q-factor of the crystal-oscillator is not reduced, preventing stopping of oscillation. Also, the crystal-oscillator circuit of the present invention forms a Colpitts oscillator circuit that prevents the stopping of oscillation.